Always return blank scene when communal is not available

The glanceable hub can be disabled for many reasons, such as the flag
being turned off or the setting being turned off. Before this change,
CommunalRepository was the source of truth of the desired scene state,
and anyone observing isCommunalShowing through the interactor would see
it incorrectly returning that communal was showing.

This change makes the interactor always return Blank for the scene in
desiredScene when communal is not available. This prevents clients from
seeing an incorrect communal state.

Bug: 322549132
Bug: 325868439
Test: atest CommunalInteractorTest MediaHierarchyManagerTest
Flag: ACONFIG com.android.systemui.communal_hub STAGING
Change-Id: I1b5c7eb81cadb44a7e10135ea92a063589580be9